Broad Band Mid-IR Transmitting

1. Broad Band Mid-IR Transmitting Single Mode Fibers (SMFs) and Integrated Optical Circuits (IOCs) - Spatial Filters for the ESA DARWIN Project Abraham Katzir Tel Aviv University, Tel Aviv, ISRAEL www.tau.ac.il/~applphys katzir@post.tau.ac.il

2. The TPF and the Darwin projects Nulling interferometry Spatial & modal filtering Single mode fiber as a modal filter Silver halide material and fibers Single mode silver halide fiber Measurements & results Micro-structured fibers Single mode flat waveguide (for Integrated Optics Circuits) Conclusions Summary Lecture Outline

3. DARWIN and TPF projects Performing atmosphere spectroscopy in the 8-20μm mid-IR spectral range for planets near stars. Indications for the presence of life? Target: Problem: A star “masks” the radiation from a neighboring planet Solution: Nulling interferometry

4. TPF and DARWIN basic idea Selecting the operating region 4µm - 20µm Nulling Interferometry

6. Collaboration & Funding Darwin - Alain Leger, Paris Pierre Kern, Grenoble TPF – Peter Lawson, Alex Ksendzov JPL

7. Wave front (phase) deviations Phase deviations caused by: A. Dust B. Telescope imperfections C. Telescope pupil Result: Destroying the interference pattern Proposed Solutions: A. Spatial filtering (Pinhole) B. Modal filtering (Single mode fibers or waveguides)

8. d 2ρ z0 Modal filtering using Single Mode Fibers Reflecting surfaces

9. Spatial Filter for the Nulling Interferometer IR Transmitting Single Mode Fibers Fold Mirrors Beam splitter Space Telescope Compensation Plate ( phase shift) Fold Mirror IR Detector

10. Theoretical evaluation of the modal filtering by a step index single mode fiber * *O. Wallner et. al.

11. a b Step index fiber configuration Real fibers: b - finite Theoretical model: b → ∞

12. Single Mode Conditions Waveguide parameter - Number of modes - Single mode condition (LP01) V<2.405 • Small difference between indices of refraction • Small core diameter

13. Modal filtering is length dependent !! *Theoretical evaluation of the minimal filter length - z0 *O. Wallner et. al.

14. 2ρ z0 Theoretical Estimates - O. Wallner et. al. Attenuation Factor – Model Definition: A= PLP0 1 (z0)/ PLM (z0) For modal filtering: • A= 106 • Filter losses ~ 1-2 dB/m

15. IR Transmitting Materials Silica Glasses Sapphire Fluoride Glasses Most Suitable Chalcogenide Glasses Silver Halide Crystals 0.1 1 10 m Wavelength [ m]

16. Candidates for Single mode fibers (Other than Silver Halides) Chalcogenides* glasses seems to have the most promising performance Developed by the University of Rennes France Under DARWIN contract Chalcogenides * Proc. SPIE 5905, 447, 2005 * J. Opt. Adv. Mat. 4, 665, 2002 Fluorides

17. Silver Halide Crystals and Fibers at Tel Aviv University (TAU)

18. Silver Halides Crystals - Optical Properties - Transmission Range AgCl 0.4 to 25m AgBr 0.45 to 35m

19. Crystal Growing System

20. AgClBr Crystals Typical Dimensions cm

21. Press Rod Heaters Upper & LowerPlates Crystal Die Fiber Extrusion of a Silver Halide Fiber

22. Silver Halide Unclad Fibers – Properties Polycrystalline Structure – Typical Grain Size ~ 1µm

23. * Measured by FTIR Silver Halide Unclad Fibers – Properties Transmission Range & Loss Coefficient* Rayleigh Gans scattering λ≈Dscat ; Iscat αλ2

24. * Measured at TAU where x – the molar fraction of chlorine in the compound. Silver Halides Crystals - Optical Properties - Refractive Indices of AgClxBr1-x Solid Solutions *

25. Summary of silver halide fiber parameters Spectral range 2 - 25 μm Optical losses at 10 μm unclad0.2 dB/m(or 95%* per meter) core/clad~1 dB/m(or 93%* per meter) core diameter unclad 0.7 - 0.9 mm core/clad0.3 - 0.6 mm Length 2 - 10 m Field of view ~ 45º Flexible, Non toxic, Non-hygroscopic, Biocompatible

26. Single Mode Fibers (SMFs) - Basic “theoretical” demands - A. Small difference between indices of refraction ≤ 2.405 B. Small core

27. Predicted Region for Single Mode Operation @ 10.6m AgClBr single mode fibers applicable for nulling interferometer mission

28. Silver halide AgClxBr1-x Single Mode Fiber (SMF) configuration a b x x+0.02 60µm>2a>50µm 2b=900µm

29. Reduction of n=n1-n2 to n~ 0.005  Improvement of the core-clad interface: - Reducing the roughness - Reducing the impurities  Reduction of core diameter to 2a ~60 - 30m  Solving the problem of cracks  Silver Halide SMF - Practical demands for single mode operation - Small Δn = Homogeneous crystals: Small core = Extrusion process:

30. Crystal Homogeneity: Crystal Growing Crystal Composition Measurements

31. [mm] 65 83.5 ± 0.8 181 82.5 84.0 52 Lower layer 82.5 84.5 6 83.0 84.5 84.0 10 84.0 41 The Composition as a Function of Position in Various Cross Sections Along a Vertical Line FOR EXAMPLE Nominal composition: 83% Br

32. Reduction of Core Diameter  Round (±5%) and homogeneous cores Smooth Interface;  Rz~200-250nm (Former Rz~1 to 2µm) α[dB/m]= 0.5 (2a=350µm), 1(140µm), 4 - 5(60µm) Measurements at =10.6m  60 m core fiber Core : AgCl40Br60 Clad : AgCl95Br5 M500 M50 900 m 60 m

33. IR Problem: Clad modes interfere with core radiation Output end of the Step Index (SI) core-clad silver halide fiber of length L=50 cm and core diameter 2a = 60m Significant total energy in the clad

34. Removal of Clad Modes Goal: Attenuation of clad modes  40dB Method: Adding an absorbing layer on the external surface of the fiber

35. a b Absorbing layer Clad mode attenuation by Application of an absorbing layer IR 900 m Output end of a coated SI core-clad silver halide fiber (comment: photograph overexposed) Core diameter = 60m

36. Optical Properties of Silver Halide Single Mode Fibers

37. SMF With Core Diameter = 50µm Composition: Core:AgCl0.3Br0.7 Inner clad:AgCl0.32Br0.68 “Smooth” far field pattern - Typical Losses  15-20 dB/m Far field distribution (L=50cm)

38. Radialfar field distribution Typical far field pattern of a 50µm core Silver halide SMF, L=50cm V# =2.1033

39. Spiricon IR camera Demonstration of modal filtering Lens SMF L=50µm Silicon windows CO2 laser

40. Microstructured Optical Fibers J. C. Flanagan et al. Main claim: Microstructured fibers are potentially better suited for modal filtering than step index (SI) fibers

41. Applied Physics Group n2 n1 C B D Photonic Crystal Fibers - PCFs n2<n1 A schematic drawing of a configuration of a TIR - PCF Transmission via Total Internal Reflection - TIR

42. Applied Physics Group PCF Thermal Camera Laser CO2 A Thermal Image of a CO2 Laser Beam Transmitted through a large core PCF Input Output Beam confined to the core area

43. Flat Waveguide Y coupled waveguides will be the basis of integrated optical circuits > 20m core thickness x~5%

45. Single Mode Flat Waveguide Thermal image of the output end of the waveguide The input end was illuminated by a CO2 Laser radiation* * Radiation was coupled directly to the flat guide, using a F= 36cm lens (D=2.54cm).

46. Discussion The extrusion process has been improved  We have developed a new crystal growing technique ensuring composition homogeneityof about ±1%  We established special extrusion conditions needed for the extrusion of core-clad fibers of extremely small cores.  We have developed an absorbing coating that is useful for stripping of cladding modes.  We have developed and fabricated fibers having small core and small n that exhibit Single Mode properties. 

47. Discussion We have developed microstcutured fiber and demonstrated transmission through its core.  We have developed a new single mode flat waveguide which can be used for fabrication of integrated optical circuit. 

48. Conclusions We have managed to develop for the first time a silver halide single mode fiber having loss between 10 to 20 dB/m @ 10.6µm We have managed to develop for the first time a silver halide flat waveguide